In printed wiring boards and multiple-pin ball grid array (BGA) packages, it is increasingly required that the wiring density be increased. Increased density requires that the interval of a conductor pattern that is formed on a substrate becomes very small. For example, about five years ago, the width of wiring lines themselves was about 150 microns (.mu.m) and the pitch of pads was about 650 .mu.m. However, at present, the width of wiring lines and the pitch of pads are less than 75 .mu.m and less than 200 .mu.m, respectively. There even exist such types that the line width is about 50 .mu.m and the pitch is about 125 .mu.m. As such, the wiring density has tripled or quadrupled in these several years. A technology is now required which accommodates the above trend.
The core of the technology for accommodating the increase in the wiring density relates to the formation of wiring layers themselves. Along with this, a decreased pitch of the wiring layers requires development of a method for forming a precision pattern of insulating layers, made of, e.g., organic material, around the wiring layers.
In a conventional technique for shaping an organic material layer into a given pattern, first a resin layer is formed by applying a liquid ultraviolet-curable resin (commonly referred to as a photoresist or resist) to a substrate, and then unnecessary portions of the resin layer are removed by exposure and development with the use of a certain mask. This technique is used very widely even today, because it can provide the highest productivity and form a precision pattern to a certain extent.
However, to attain a small pitch that is in the above-mentioned range, there occur problems with currently available ultraviolet-curable resins. The problems are associated with the resist (a specific example is a resist that reacts and sets upon illumination with ultraviolet rays; also called "solder resist"). The resins most commonly used for industrial purposes as resists are required to satisfy not only a high resolution property but also other various properties, which are opposed to those required for high resolution. For example, for a solder resist or the like to become a permanent protection film it should be highly reliable. The reliability, which is a very broad concept, is often defined as a collective concept including a low possibility of occurrence of diffusion phenomena such as migration between wiring layers and good adhesiveness with a substrate. Many other properties required include high mounting performance and high adaptability to a manufacturing process (specifically, a low possibility of occurrence of solder balls due to post-flux repulsion and high adhesiveness with a sealing resin for fixing mounting parts). The ultraviolet-curable resist might accommodate the above-mentioned pitch reduction if the high precision property were the only requirement. However, actually the ultraviolet-curable resist is required to satisfy a variety of conflicting properties, and currently there does not exist a single ultraviolet-curable resist which satisfy all of the properties required. Problems associated with a variety of properties required for the ultraviolet-curable resist will be described later in terms of specific problems with formation of resist dams.
The resist dam is a post-shaped portion made of an insulating material such as a resist. The resist dam is formed in the vicinity of a conductive pad formed on a substrate, and serves as a dam for preventing solder on the conductive pad from contacting with an adjacent conductive pad. That is, the resist dam is a dam made of a resist which serves to prevent solder from short-circuiting adjacent wiring conductors. In other words, the resist dam has a role of preventing the occurrence of what is called solder bridging. The technique itself of preventing solder bridging by means of a dam of an insulative layer is known as disclosed in Published Unexamined Utility Model Application No. 57-163764. This utility model application discloses a structure which prevents short-circuiting of conductive members by forming dams of insulative layers in the vicinity of the conductive members.
A technique of patterning an insulative layer at a fine pitch is necessary to forming resist dams. FIG. 1 shows a general appearance of resist dams. FIG. 1 is a top view of wiring conductors. As shown in FIG. 1, wiring conductors 2 are formed on a substrate and an insulating layer 3 is so formed as to generally cover the entire structure. Windows 4 are formed in the insulating layer 3 immediately above pads 2a (that are tip portions of the wiring conductors 2) to thereby partially expose the pads 2a. This becomes more apparent by referring to FIG. 2(a), which is a sectional view taken along line 2a--2a in FIG. 1. Referring to FIG. 2(a), the wiring conductors 2a are formed on the substrate 1 and resist dams 3a are formed between the wiring conductors 2. It is seen from comparison with FIG. 1 that the resist dams 3a constitute part of, that is, continuous with, the insulating layer 3 and are defined by the windows 4. As a matter of fact, when solder 5 is formed on the pads 2a, it is actually formed only on the pads 2a by virtue of the existence of the resist dams 3a. If it were not for the resist dams 3a, solder formed on one pad 2a may contact with the adjacent pad 2a, to cause short-circuiting. This phenomenon is called solder bridging.
FIGS. 3-6 show a background art method for forming resist dams. As shown in FIG. 3, a number of pads 2a are formed on the substrate 1 in a predetermined pattern. The substrate may be any of a ceramic substrate, a glass substrate, and an organic resin substrate. Next, as shown in FIG. 4, a liquid resist 3 of an ultraviolet-curable type, which is on the market, is applied to the substrate 1 and the pads 2a. Next, as shown in FIG. 5, the liquid resist 3 is exposed to ultraviolet rays with the use of an exposure mask 7. In this operation, openings (hatched portions) of the mask 7 are positioned immediately above the pads 2a. Thereafter, as shown in FIG. 6, portions of the liquid resist 3 are removed by development. As a result, resist layers remain at desired positions to form resist dams 3a between the wiring conductors 2a.
The use of this resist dam technique to fine-pitch wiring conductors is disclosed in prior applications. For example, Published Unexamined Patent Application No. 2-272791 discloses a structure in which solder resists are provided in the spaces between wiring pads and at least a part of pad peripheries that are continuous with the spaces in a case where the spaces between the pads are too narrow to form resist dams. In this application, because of the narrow spaces between the wiring pads, the resist dam formation itself is replaced by another technique. Further, Published Unexamined Patent Application No. 6-252540 discloses a technique of making resist dams that are formed in the vicinity of metal pads higher than the total height of the pads and solder formed thereon. Higher resist dams would be somewhat effective in preventing solder bridging in forming fine-pitch wiring conductors. However, this application does not disclose any technique of effectively forming dams of insulative layers at very narrow gaps between the fine-pitch wiring lines, and therefore cannot be considered a proposal of an essential solution.
As the pitch of wiring conductors is reduced, the problem of migration between the wiring conductors becomes more likely to occur. Migration is a phenomenon in which metal that constitutes a wiring layer is ionized and resulting ions diffuse through a space between insulating layers to finally reach the adjacent wiring layer. Since migration is a diffusion phenomenon, the problems become more serious as the space between wiring conductors is reduced. In general, the diffusion phenomenon occurs between adjacent wiring conductors (see pitch "W" between 2 and 2' in FIG. 1). The migration mechanism becomes apparent by referring to FIG. 2(b), which is a sectional view taken along line 2b--2b in FIG. 1. In the 2b--2b cross-section, the wiring conductors 2 and 2' are completely covered with the insulating layer 3. Migration occurs between the wiring conductors 2 and 2' through the insulating layer 3. In particular, at a present time when the pitch W of the wiring conductors is small (less than 125 .mu.m), the migration problem comes to exist. One method for preventing this problem is to study the material of the insulating layer 3. One countermeasure would be to select an ultraviolet-curable resist that is highly resistant to migration, but this type of resist is generally low in resolution property. If a resist having a high resolution property is used to attain a fine pitch, the migration resistance is lowered. As such, the resolution and the migration resistance are contradictory properties. This results from the fact that a photo-initiator, a sensitizer, and an amine-type substance which are typically added to increase the resolution of an ultraviolet-curable resist also have effects of increasing free ions in the resin and enhancing the permeability of chlorine ions and alkali ions that are contained in chemicals used in a manufacturing process, thereby making the diffusion phenomenon more active through interaction with moisture, that is, adversely affecting the migration resistance property. Thus, to provide a single ultraviolet-curable resist that satisfies a plurality of contradictory properties is a subject to be attained in connection with the current ultraviolet-curable resist.
More detailed discussions will be made of the relationship between increasing the density of wiring conductors and the migration resistance. In particular, the requirement of increasing the density of wiring conductors is converted into a requirement for a property of the resist, that is, high resolution of a photo-curable resist used in a manufacturing process. On the other hand, the migration resistance is required for a resist as a constituent material of a completed product. As described above, if a resist is used which enables precision patterning to accommodate the pitch reduction, the migration resistance is lowered. Therefore, with the tendency of pitch reduction of wiring conductors, the object of forming resist dams between narrow wiring layer and the object of minimizing the migration between wiring conductors cannot be attained at the same time as long as the same insulating layer (i.e., the resist). This is one aspect of specific problems with formation of resist dams which correspond to the problems associated with a variety of properties required for the resist.
Further, the use of a resist is problematic in terms of formation of a precision pattern. For example, since the resin is in liquid form, it is applied to a substrate first. However, there may occur such defects as blooming and resin sticking in subsequent exposure and development steps. The occurrence of these types of defect is particularly troublesome in forming a precision pattern. This is because in forming a precision pattern, small pattern dimensions more likely cause exposure defects and misregistration. However, since the resin has already photoreacted at a time point when such defects are found, it is difficult to strip off the resin from the substrate, which means that there is no other way than scrapping that substrate. This results in a decrease in production yield.
Thus, if resist dams are formed by using an ultraviolet-curable resist, the production yield will become low though resist dams of a very high precision pattern can be formed. In addition, there is a possibility that quality-related problems such as migration will occur in products.
On the other hand, it is possible to form resist dams by using a thermosetting resist. Thermosetting resists are less likely to cause inferior migration resistance and yield reduction, which are problems associated with ultraviolet-curable resists. However, thermosetting liquid resists are not suitable for forming of resist dams of a precision pattern which is intended by the invention, because they are low in resolution.
The industry seeks a method of making an insulating layer overcoming at least some of the aforementioned short comings.